1. Field of the invention
The present invention relates generally to semiconductor fabrication methods and, more particularly, to a photomask with illumination control for controlling the intensity of light that is passed through a plurality of areas on the photomask having different structural densities.
2. Description of Related Art
An almost unbridled movement in the semiconductor industry toward augmented processing power continues to spurn the industry-wide movement toward greater device performances and densities. In seeking to achieve these ends, the transistors used to form integrated circuits are made smaller and packed denser to increase the overall performance of the device. Regarding device densities, current technologies allow for the creation and positioning of over a billion transistors on the surface of a semiconductor, and this number is likely to continue to increase in the future. As devices are developed, the precise positioning and uniformity of the components that form the transistors continues to be an important feature in the manufacturing process of integrated circuits.
As a result of the greater number of transistors in modern integrated circuits, multiple interconnect layers are implemented to electrically connect the transistors. An interconnect layer can comprise very thin conductive lines precisely positioned and surrounded by a dielectric material to prevent cross-talk within the interconnect layer. Neighboring interconnect layers are separated by an interlayer dielectric layer in an effort to attenuate cross-talk between interconnect layers.
Within a semiconductor integrated circuit, the conductive lines in the multiple interconnect layers are typically connected to one another through the use of via. A via is a precisely positioned aperture formed in the interlayer dielectric that is filled with a conductive material for the facilitation of an electrical interconnection between the conductive lines in the interconnect layers.
One of the processing steps used during the manufacturing process of transistors, conductive lines and via is photolithography. Photolithography is used numerous times during the manufacturing process and is one of the more important as well as one of the limiting processes for determining the maximum density and final reliability of the integrated circuits. Photolithography is particularly important in positioning the transistors, interconnect layers and via and in ensuring their uniformity. Current photolithography technologies allow for the manufacture of devices with features smaller than 0.25 micron. The size of the possible features will undoubtedly continue to shrink in the future as photolithography processes continues to improve.
A typical photolithographic process is implemented by depositing onto a working surface, by some means (usually a spinner), a layer of photosensitive resist which can be patterned by exposure to ultraviolet (UV) light or another radiation type. The working surface may be a semiconductor wafer, interconnect layer or other layer depending on the current manufacturing stage of the integrated circuits. The photoresist layer is sensitive to light and may be patterned based on exposing the photoresist to a corresponding pattern of light. The frequency of the light can be very important in determining the size of the integrated circuit features which may be created. Smaller features may be made by higher frequencies (shorter wavelength) light sources. Current efforts are being expended to develop 248 nm (KrF excimer laser lithography), 13.4 nm (extreme ultraviolet lithography), and 1.0 nm (X-ray lithography) wavelength light lithography systems.
When exposed to light, photoresist may either be hardened or softened, depending on the type of photoresist used. Positive photoresist, also known as light-softening photoresist, can be depolymerized by exposure to radiation such as UV light. Therefore, with positive photoresist, areas exposed to radiation are dissolved upon placement in a developer, while the masked, unexposed areas remain unaffected. On the other hand, negative photoresist, which is a light-hardening photoresist, can be polymerized by exposure to radiation, meaning that the exposed areas remain, while the covered areas are dissolved. Thus, depending on the type of photoresist utilized, the pattern transferred to the photoresist on the wafer is either a positive or a negative image of the photomask pattern.
To undergo exposure, the photoresist covered wafer is placed beneath a photomask designed to prevent the penetration of radiation through certain portions of the photoresist. Predetermined areas of the photoresist then undergo a degree of polymerization or depolymerization, which can be a function of the nature and extent of photoresist exposure to the radiation.
The photomask operates as a reticle by creating a pattern of light when it is placed between a light source and a photoresist layer spread over a wafer. The photomask forms the pattern by having areas that block the light and other areas that allow the light to pass from the light source to the photoresist layer. The pattern of light created by the photomask is typically for a single die on a wafer. A lens may be positioned between the photomask and the photoresist layer to reduce the size of the pattern and to focus the pattern of light onto the die. The lithography tool steps to the next die on the wafer and repeats the process until all the die on the wafer have been exposed to the pattern of light created by the photomask.
A chemical bath known as a developer can then be used to dissolve parts of the photoresist which remain relatively depolymerized after the radiation by placing the wafer therein and allowing the wafer to be rinsed for a designated time period. Having received the pattern from the photomask, the layer of photoresist on the wafer is typically referred to as a layer of patterned photoresist. The presence or absence of photoresist across the working surface creates a pattern or template to be used by subsequent processing steps of the integrated circuit. For example, an etching or an ion implantation process may be used after the lithography step on the exposed areas without photoresist to continue the manufacturing process of the integrated circuit.
However, conventional photolithography often overexposes and/or underexposes areas on the photoresist layer. This problem can arise when the density of features in the pattern is non-uniform across the surface of the photomask. Areas with a dense pattern (areas that tend to block more of the light) on the photomask tend to underexpose the photoresist while areas on the photomask with a less dense pattern (areas that tend to block less of the light) tend to overexpose the photoresist. Adjustments made to correct for underexposure of the photoresist layer tend to exasperate the overexposure condition and vice versa.
A need thus exists in the prior art for a photomask, having a pattern defining a plurality of areas of different structural densities, that can facilitate better control of the light intensity passing therethrough to a photoresist layer. Such a photomask should allow for the existence of arrays and mini-arrays without the need of biasing the light intensity passing to a photoresist layer with a critical dimension (CD) bias. The CD bias may create areas of under or overexposure of the photoresist layer. Such a CD bias can be difficult to control, and the degree of underexposure or overexposure may vary between processing cycles. A further need exists in the prior art for a method of manufacturing the photomask so that an unbiased light intensity can be passed through the photomask to the photoresist layer.
The present invention addresses these needs by providing a photomask, and a method for manufacturing the photomask, that allow the light intensity to be unbiased even between arrays and mini-arrays or isolated and dense regions. A photomask may thus be constructed and used that minimizes over and underexposure due to different structural densities of the pattern on the photomask without the use of optical proximity correction (OPC) or other reticle enhancement techniques (RET) which can increase the complexity and cost of the photolithography process.
In one embodiment, the photomask herein disclosed includes a transparent substrate, a slightly opaque transmission control layer and a patterned, opaque reflective layer. The patterned reflective layer creates a pattern that either blocks or permits passage of light in different areas as the light travels to the photoresist layer. However, certain areas in the pattern on the photomask may have a greater structural density of the reflective layer than other areas. It is recognized in accordance with the invention that areas on the pattern having a higher structural density of the reflective layer tend to underexpose the photoresist layer in areas of the pattern where light is desired. Likewise, it is recognized in accordance with the invention that areas on the pattern having a lower structural density of the reflective layer tend to overexpose the photoresist layer in areas of the pattern where light is desired. In accordance with an aspect of the present invention, the under and overexposure issue is addressed by the provision of a transmission control layer which can be engineered to control an attenuation of light intensity as the light passes through the transmission control layer.
According to one aspect of the present invention, an attenuation of the light intensity is controlled by altering thickness of the transmission control layer. The transmission control layer is slightly opaque and therefore attenuates the light intensity in relation to the thickness of the transmission control layer. For example, a thicker transmission control layer may be formed in areas that otherwise would be overexposed and a thinner or no transmission control layer may be formed in areas that would otherwise be underexposed.
In accordance with another aspect of the present invention, a process for manufacturing a photomask with illumination control is provided. A transmission control layer followed by a reflective layer may be formed over a substrate. A first photoresist layer may then be deposited and patterned over the reflective layer. Absence of material in the pattern of the first photoresist layer allows apertures to be etched through the reflective layer and through a first desired distance in the transmission control layer by using the patterned first photoresist layer as a mask. A second photoresist layer may be deposited and patterned over the reflective layer. Once again, absence of material in the pattern of the second photoresist layer allows apertures to be etched through the reflective layer and through a second desired distance in the transmission control layer. The thickness of the remaining transmission control layer can be formed to be greater in areas having a lower structural density of the reflective layer than in areas having a higher structural density of the reflective layer.
The pattern on the photomask is formed by the apertures that are etched through the reflective layer on the photomask. The apertures may take the form of circles or squares which may be used, for example, to create via through dielectric layers or openings for ion implantation into a semiconductor substrate. The apertures may also be formed as elongated grooves which may be used, for example, to create electrical connections within an interconnect layer. The apertures are not limited to any given shape and may comprise other shapes or combinations of shapes as is needed.
Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims.